Apple - How can I turn off iCloud storage space warning notifications?

I think your options are limited here.

1. Turn off iCloud Backup altogether.
   Some info on how to selectively or collectively switch off iCloud is here: https://support.apple.com/kb/PH2613?locale=en_GB&viewlocale=en_US

2. Back up less, e.g. by deciding not to back up your camera roll. I carefully manage what gets backed up and what not (and also which app gets mobile internet access).
   To see which apps on your phone are taking up the most space follow these steps:
   

   • Open the settings app
   • Tap iCoud
   • Scroll down to Storage & Backup and open it
   • Tap Manage Storage
   • Select your iPhone

3. If you're on Windows 10 (unlikely in this forum...), type in the Windows 10 search box Settings and select "Settings Trusted Windows Store App"
   

   • Click "System"
   • Click "Notifications & Actions"
   • Scroll down to "Show Notifications from these apps"
   • find "iCloud" and turn it off
     
     This works for other offending app notifications

Notifications pop-up when your iCloud is configured for back-up data. You can disable this and have your iPod, iPad or iPhone backed up to your computer instead.

Here:

iCloud automatically backs up your iOS device information daily over Wi-Fi when your device is turned on, locked, and connected to a power source.

Back up automatically

On your iOS 9 or iOS 8 device: Go to Settings > iCloud > Backup, then turn on iCloud Backup.

On your iOS 7 device: Go to Settings > iCloud > Storage & Backup, then turn on iCloud Backup.

Back up manually

On your iOS 9 or iOS 8 device: Go to Settings > iCloud > Backup, then tap Back Up Now.

On your iOS 7 device: Go to Settings > iCloud > Storage & Backup, then tap Back Up Now.

For information about messages that may appear while backing up, see the Apple Support article Get help backing up your device in iCloud.

Important: If you don’t back up your iOS device to iCloud for 180 days or more, Apple reserves the right to delete your device’s iCloud backups. For information, see iCloud Terms and Conditions.